The present invention is directed to an improved fundus retinal imaging system which provides high resolution multispectral retinal images over a wide field of view to permit early diagnosis of various pathologies such as diabetic retinopathy, ARMD (age related malocular degeneration) and glaucoma. More specifically, the present invention relates to a conventional fundus retinal imager combined with, inter alia, a multispectral source, a dithered reference, a wavefront sensor, a deformable mirror, a high resolution camera and deconvoluting software to produce wide field, high resolution, multispectral images of the retina.
The ability to resolve fine details on retinal images can play a key role in the early diagnosis of vision loss. Certain biochemical and cellular-scale features, which may be present in the early stages of many retinal diseases (e.g., ARMD), cannot be detected today with current funduscopic instruments because of the losses in spatial resolution introduced by the ocular medium of the eye and the lack of selectable spectral data. Additionally, the presence of aberrations within the eye limits the effective input pupil size of a standard fundus retinal imager to about 2 mm. This limit leads to a decrease in the contrast of the small image details due to diffraction effects.
A partial solution to the foregoing problems is to use an adaptive optical system, first for measuring aberrations and then for correcting such aberrations. With such a system, it is possible to increase the system pupil diameter up to 7-8 mm and achieve a resolution on the order of 10 xcexcm. The feasibility of this approach has been demonstrated recently by J. Liang et al., xe2x80x9cSupernormal vision and high-resolution retinal imaging through adaptive optics,xe2x80x9d J. Opt. Soc. Am. A/Vol. 14, No. 11/November, 1997. They report constructing a fundus retinal imager equipped with adaptive optics that permits the imaging of microscopic structures in living human retinas. The optical system, which is illustrated in FIG. 2 of this reference, includes a deformable mirror for wavefront compensation and a wavefront sensing module including a Hartmann-Shack (also known as a Shack-Hartmann; hereinafter abbreviated xe2x80x9cS-Hxe2x80x9d) wavefront sensor. Collectively, the S-H sensor, (which is used to measure the eye""s optical aberrations) and the deformable mirror (which is used to make small corrections of the optical aberrations) is sometimes referred to as an adaptive optics system. The deformable mirror is positioned in a plane which is conjugate with both the eye""s pupil plane and the front surface of the lenslet array of the S-H wavefront sensor. The S-H wavefront sensor is described in detail in J. Liang, et al., xe2x80x9cObjective measurement of wave aberrations of the human eye with the use of a Hartman-Shack wave-front sensor,xe2x80x9d J. Opt. Soc. Am./Vol. 11, No.7/July 1994. The displacement of the image, of each of the lenslets in the S-H wave front sensor, on a CCD gives information required to estimate the local wavefront slope. From the array of slopes, the wavefront is reconstructed via a least squares technique into Zernike modes. In operation, a point source produced on the retina by a laser beam is reflected from the retina and received by the lenslet array of the S-H wavefront sensor such that each of the lenslets forms an image of the retinal point source in the focal plane the CCD detector located adjacent to the lenslet array. The output signal from the CCD detector is acquired by a computer, which processes the signal and produces correction signals which, via a feedback loop, are used to control the deformable mirror.
There are a number of limitations associated with the above described instrumentation including:
1. Sensitively to speckle modulation within the eye;
2. The deformable mirror can only provide limited correction;
3. It is a panchromatic instrument, not multispectral;
4. It operates with a limited field of view, on the order of 2-5 degrees; and
5. Several renditions of the S-H output are required to estimate the wavefront.
Further, while it is claimed that it is useful in determining aberrations beyond defocus and astigmatism and providing improved imaging inside of the eye, there is no discussion of its use as a clinical instrument to be used in the diagnosis of the major causes of vision loss and blindness. Finally, the Liang et al. instrument is a laboratory device composed of very expensive one-of-a-kind components.
It is an object of the present invention to provide, in association with any commercially available fundus imager, an improved, low bandwidth adaptive optics system and an optimized depth sensitive deconvolution technique to increase retinal imaging resolution and field of view, to thereby enable a clinical device to improve the level of opthmological healthcare.
It is another object of the present invention to provide a deconvolution technique which takes into account the reflectance of difficult colors from the various layers of the retina to provide a high spatial resolution, multi-spectral image over a wide field of view.
It is another object of the present invention to provide a fundus based opthalmic instrument which has resolution at the micron level (i.e., less than the size of a cell).
It is yet another object of the present invention to provide a retinal imaging system which uses a scanning (or dithered) reference spot to mitigate the speckle problems associated with the instrumentation disclosed by Liang et al. and which allows wavefront estimates and images of the retina to be taken with one exposure instead of multiple exposures.
It is yet another object of the present invention to correct for large, low order aberrations (e.g., tip, tilt, focus, and astigmatism) using a bimorph adaptive optical element.
It is yet still another object of the present invention to use post image depth sensitive deconvolution techniques to correct for high order aberrations (e.g., coma, trifocal, spherical, and higher terms) and remove residual low order aberrations.
It is yet a further object of the present invention to provide the foregoing in an affordable attachment to existing fundus retinal imagers.
It is yet another object of the present invention to accomplish the wavefront sensing using a distorted grating based wavefront sensor instead of the S-H sensor.
The foregoing and other objects will be apparent from the disclosure which follows.
An ophthalmic instrument having a wide field of view (up to 20 degrees) including a retinal imager, (which includes optics for illuminating and imaging the retina of the eye); apparatus for generating a reference beam coupled to the imager optics to form a reference area on the retina; a wavefront sensor optically coupled to the imager optics for measuring the wavefront produced by optical aberrations within the eye and the imager optics; wavefront compensation optics coupled to the imager optics for correcting large, low order aberrations in the wavefront; a high resolution detector optically coupled to the imager optics and the wavefront compensation optics; and a computer (which is connected to the wavefront sensor, the wavefront compensation optics, and the high resolution camera) including an algorithm for correcting, small, high order aberrations on the wavefront and residual low order aberrations. The wavefront sensor includes a Shack-Hartmann wavefront sensor having a lenslet array and a detector positioned in the front surface of the lenslet array for producing a Hartmannogram. Alternately, the wavefront sensor is a distorted grating wavefront sensor, including a distorted grating, an imaging lens, and a detector positioned in the focal plane of the imaging lens. The computer includes means for estimating the wavefront from the Hartmannogram or distorted grating image and sending a correction signal to the wavefront compensation optics to correct large, low order aberrations in the wavefront. Only one Hartmannogram, or one distorted grating image, is required, thereby reducing the exposure of the retina to the spot, and avoiding the need to register successive Hartmannograms or distorted grating images. The wavefront compensation optics includes a deformable mirror, such as a bimoph mirror. The algorithm for correcting small, high order aberrations includes a deconvolution algorithm which utilizes information from both the wavefront sensor and the high resolution detector. The deconvolution algorithm includes an algorithm for estimating the wavefront sensed by the wavefront sensor, means for estimating the Optical Transfer Function of the wavefront, and Weiner Filter Estimation means. The deconvolution algorithm also includes image reconstruction algorithms. The instrument also includes a plurality of filters and the deconvolution algorithm also accounts for the reflectance of various wavelengths of light from different depths within the retina to produce a multispectral deconvoluted image of the retina. The instrument also includes a mechanism for dithering the reference beam, including a rotatable wedge. Because the instrument produces a wide field of view, a large format, high-resolution detector is required. The instrument, less the retinal imager, is an adaptive optics system which can be used in association with a number of commercial imagers, including fundus imagers.
A method of obtaining high resolution, wide field of view, multispectral images of the retina from the apparatus of the present invention.
These and other objects will be evident from the description that follows.